1. Field of the Invention
This invention relates to the generation of radiation at a desired wavelength.
More precisely, this invention relates to a process for the generation in a given direction of radiation emissions in a desired range of wavelengths, where the process includes: producing initial radiation by a radiation source whose wavelengths includes the desired range; and filtering the initial radiation, to substantially eliminate the part of the initial radiation whose wavelength is outside the desired range.
The invention also relates to a radiation generating device that can be used to implement such a process, as well as a lithography device incorporating such a generation device.
2. Discussion of Related Art
Processes and devices similar to the type mentioned above are known.
One (non-limiting) example of implementation concerns the generation of radiation at a desired wavelength intended for an optical chain, for lithography applications on a photosensitive substrate. FIG. 1 diagrammatically illustrates an optical system 100, that includes the following in succession:
a generator 10 of radiation in a desired range of wavelengths;
a lens assembly 11 which receives the radiation coming from the generator 10 and processes it (e.g., by subjecting it to collimation and/or focusing of its beams);
a mask 12 which receives the processed radiation coming from the lens assembly 11, and selectively allows to pass only the beams arriving at the mask via a transmission pattern 120, with the remainder of the radiation being stopped by the mask; and
a substrate 13 which receives the beams that have been transmitted by the mask, and whose surface exposed to the radiation bears a photo-resistant or photosensitive product.
The beams arriving at the substrate react with the product and thus form, on the surface of the substrate, a pattern that matches the transmission pattern of the mask.
The desired range of wavelengths of the generator 10 can in particular be located in the ultra-violet (UV) spectrum, or in the extreme UV (EUV) spectra.
Note that in this text, the term “EUV” is conventionally used to refer to both EUV beams and soft x-rays.
The EUV beams are associated with very short wavelengths (wavelengths less than 100 nm, and of the order of a few tens of nms for example, where an application corresponds to a wavelength of 13.5 nm). This is advantageous in particular for photolithography applications, since in a corresponding manner, the patterns drawn by the beams can be of very small dimensions. In particular, this allows the formation of a larger quantity of patterns on a substrate of the same size.
It is necessary however to associate radiation filtering resources with the radiation generator.
In certain embodiments, in particular for radiation generators whose wavelength is in the EUV range, the generator includes a radiation source of the plasma source type.
In addition to the desired radiation, such radiation sources also emit: radiation whose wavelengths do not correspond to the desired range; and/or solid debris resulting from the interaction between the plasma and solid parts of the chamber in which this plasma is located (target, walls of the chamber, etc.).
In order to isolate, in the radiation coming from the source of the generator, only the beams that are at a desired wavelength, it is therefore necessary to provide filtering resources downstream of the source (e.g., immediately downstream of the source, in order to avoid exposing the mask to debris which could damage it).
In a known manner, such filtering resources include a multi-layer mirror which selectively reflects the beams according to their wavelength. Such a multi-layer mirror thus functions as a band-pass filter. Such a multi-layer mirror does not pass on the undesirable debris which can be emitted by the source, so that the elements located downstream of the filtering resources are not exposed to such debris. Such a solution allows filtering out of the beams emitted by a radiation source that can produce such debris. However, one drawback associated with such a known configuration is that the debris emitted by the source can damage the mirror of the filtering resources.
It would be possible to envisage distancing the filtering resources from the source, so as to reduce the probability that debris will damage the mirror of these filtering resources.
In this case however, there would be a significant reduction in the radiation stream recovered by the filtering resources, thus adversely affecting the performance of the whole optical system.
It therefore appears that the known configurations for generating radiation at a desired wavelength are associated with drawbacks when the radiation source can generate debris.
In particular, this disadvantage concerns applications in which the desired wavelengths fall in the EUV area.